1. Field of the Invention
The present invention relates to threshold circuits and in particular to threshold gates suitable for operation with subnanosecond pulses.
2. Description of the Prior Art
A threshold gate is an electronic circuit receptive of input signals for producing an output signal at one value so long as the input signal is below some predetermined value. When the input signal rises above the predetermined value, the output signal changes to a different value within some time, .DELTA.t, thereafter and remains there for a predetermined time. Many such threshold circuits exist where .DELTA.t is in the nanosecond range or greater. Few threshold gate circuits exist, however, for use with input signals in the subnanosecond range. The principle example is the avalanche transistor. G. F. Ross in an article entitled "The Accurate Measurement of Range by the Use of Microwave Delay Line Techniques," in IEEE Trans. on Micro-Wave Theory and Tech., vol. MTT-23, No. 12, Dec. 1975, pp. 1071-1075, has demonstrated threshold circuits constructed with avalanche transistors. These devices have large package delays and poor threshold characteristics for some applications.
A transferred-electron device with a Schottky-barrier gate known as a TELD has excellent threshold properties and is capable of operation at frequencies having a subnanosecond period. A TELD is not, however, well suited to drive low impedance loads while providing gain. Although an impedance matching transformer can be designed to operate at any given frequency, it does not operate as required for fast rise time pulses. Therefore, an impedance matching transformer is not suitable for cascading in a TELD gate circuit to drive low impedance loads.